Recently, in the industry of welding, more effort into improvement of productivity is continued every day. Particularly, requests to reduce a short-time stop of a production line caused by minor troubles and to reduce a tact time become higher than before.
Various factors are thought as causes of the stop of the production line. The largest cause is a trouble caused by poor arc start.
Therefore, in the arc start in a conventional consumable electrode type welding process, when a start signal is input from the exterior part, a robot manipulator is driven to move a welding torch to a weld-start-position which has been previously taught. Thereafter, in a state where feeding of a wire is stopped, the welding torch is moved by the robot manipulator nearly in the feeding direction of the welding wire and a leading end of the wire is gradually brought close to a workpiece. When it is judged that the wire leading end has come into contact with the workpiece, an initial current of a predetermined small current value is applied by a welding power supply unit. Simultaneously, the welding torch is moved in the opposite direction to the feeding direction of the welding wire thereby to perform a retreat movement in which the wire leading end is kept away from the workpiece. When the wire leading end is separated from the workpiece by the retreat movement, an arc to which the initial current connects is generated. In a state where the initial arc generating state is kept, the retreat movement is continued thereby to return the welding torch to the weld-start-position. Thereafter, the retreat movement is switched to movement in the welding direction which has been previously taught, and simultaneously the feeding of the welding wire is started and a steady welding current is applied, whereby the initial arc generating state is shifted to the steady arc generating state (refer to, for example, Patent Document 1).
FIG. 4 is a whole block schematic diagram of a welding system by means of a robot that uses the above consumable electrode type welding process.
In FIG. 4, a welding wire 101 that is a consumable electrode is drawn out from a wire spool 102 in the direction of a welding torch 104 by a wire feeding motor 103.
A welding power supply unit 105 applies the predetermine welding current I and welding voltage V between the welding wire 101 and a base material 107 that is a workpiece through the welding torch 104 and a welding tip 106, thereby to generate an arc 108 and control the wire feeding motor 103.
A robot manipulator 109 holds the welding torch 104, locates the welding torch 104 at a weld start position (not shown), and moves the welding torch 104 along a weld line (not shown).
A robot controller 110 performs bilateral communication S with the welding power supply unit 105, and transmits welding conditions such as the welding current I and the welding voltage V, and weld start and end commands, thereby to control the robot manipulator 109.
A consumable electrode type welding process in the thus constructed welding system will be described with reference to a time chart of FIG. 5.
In FIG. 5, a vertical axis represents each condition of a traveling velocity TV of the welding torch, a feeding velocity WF of the welding wire, a short-circuit judgment signal A/S, a welding current I, and a welding voltage V; and a horizontal axis represents time. A point of time when a weld start signal has transmitted from the robot controller 110 to the welding power supply unit 105 is denoted by TS0′, and TS1′ to TS5′ after the TS0′ will be described later.
Firstly, the robot controller 110 transmits a weld start signal to the welding power supply unit 105, actuates the robot manipulator 109, and causes the welding torch 104 to accelerate toward the base material 107. When the velocity of the welding torch 104 comes to an initial torch velocity TV0, the acceleration of the robot manipulator 109 is stopped, so that descent of the welding torch 104 continues at a constant velocity.
Further, upon reception of the weld start signal from the robot controller 110, the weld power supply unit 105 applies a no load voltage V0 between the welding wire 101 and the base material 107.
When the welding wire 101 and the base material 107 contacts each other at the time TS1′, a short-circuit judgment signal A/S is outputted from a short-circuit judging unit (not shown) provided in the welding power supply unit 105.
When this short-circuit judgment signal A/S is transmitted to the robot controller 110 through the bilateral communication S, the robot controller 110 decelerates and stops immediately the robot manipulator 109. At the time TS2′, the operation of the robot manipulator 109, that is, the velocity of the welding torch 104 comes to zero.
Thereafter, the robot controller 110 reverses the operation of the robot manipulator 109 immediately, whereby the welding torch 104 starts an operation in a direction separating from the base material 107 to perform a pull-up operation of the welding torch 104.
An interval between the time TS1′ and the time TS3′ is an initial short-circuit period. From the TS1′ to the time TS2′ when the robot manipulator 109 decelerates and the velocity of the welding torch 104 comes to zero, the robot manipulator 109 presses the wire 101 against the base material 107. From the time TS2′ and on, the operation of the robot manipulator 109 is reversed, so that the pressing amount of the wire 101 decreases gradually, and at a point of the time TS3′, the short-circuit is released.
The time TS3′ when this short-circuit is released occurs when the area of a triangle cde that is formed by a line TV representing the velocity of the welding torch 104 and represents the pull-up amount of the welding wire 101 is greater than the area of a triangle abc that represents the pressing amount of the welding wire 101.
Further, the welding power supply unit 105, when the initial short-circuit occurs at the time TS1′, controls the welding current I at a welding current I1′. A predetermined time later, the welding power supply unit 105 increases the welding current I1′ to a current I2′, and waits for short-circuit release.
The reason why the welding current is controlled at I1′ that is set comparatively low in a first stage in this initial short-circuit period is that: it is prevented that the welding wire 101 melted by the Joule heat of the leading end portion of the welding wire 101 generated due to the initial short-circuit scatters simultaneously with generation of the arc thereby to become spatter.
Further, the reason why the current I1′ is changed to the current I2′ is to given energy enough to generate an arc at the short-circuit release time of the time TS3′.
When the arc is generated at the time TS3′, the welding power supply unit 105 starts the wire feeding motor 103 and accelerates the welding wire 101 toward the base material 107. The acceleration is continued till the velocity of the welding wire 101 comes to a welding wire velocity for regular welding. After the velocity of the welding wire 101 has come to the welding wire velocity for regular welding, the feeding of the welding wire 101 is continued at a constant velocity.
Further, the welding power supply unit 105 controls the arc current I at an arc initial current I3′ for a fixed time in synchronization with starting of the wire feeding motor 103, and thereafter controls the arc current I at a second initial current I4. Thereafter, the welding power supply unit 105 controls the arc current I at regular welding output (not shown).
Patent Document 1: JP-A-2002-205169